Use of brushless DC motors is growing faster than any other motor type. Their application in blowers is no exception. Among the many benefits these motors offer is the ability to finely control speed — everything from fixed speed to blowers run under pulse-width modulation (PWM) for full control of direction and rpm.
A brushless motor accepts electrical input in order to output mechanical energy; the electromagnetic field of its permanent magnets interacts with that created by switching (commutating) electrical current through windings to create torque.
One variation of a permanent-magnet motor is an external rotor motor, in which it includes a cup-shaped rotor with permanent magnets affixed to its inner diameter. These external rotor motors — also called cup motors or outer-rotor motors — include a hollow rotor that has magnets on its I.D. This cup rotor spins around a wound stator that accepts input electrical power often shaped to deliver a set speed.
In many of its blowers, AMETEK Dynamic Fluid Solutions (DFS) uses cup motors. Cup motors excel in blower designs because they’re compact, especially where the external rotor assembly does double-duty as the blower impeller. The design also facilitates motor-winding heat removal, in some cases with a dedicated fan attached to the motor rotor, and separate from that for the working application. One caveat here is that with such integration and the high speeds at which blowers operate, precision balancing of the motor-rotor output assembly is necessary.
Perhaps more significant is the higher inertia of the cup motor design — an inherent design characteristic that can:
- Help blowers steadily output torque even through loading variations — and deliver higher power density because of a larger working circumference.
- Minimize torque ripple and noise, boosting low-speed open-loop performance.
But there’s another effect of rotor inertia. Just consider the mechanical time constant for a blower, which is the time required in seconds for the blower-motor speed to reach 63.2% of its final value for a fixed voltage. Rotor inertia J (expressed in kg-m2) is a dominant factor in the equation. The mechanical time constant is calculated as:
motor rotor inertia resistance / motor torque constant motor back EMF
This value is important because when a blower fails to reach full speed within the time required by the application (or at all) it’s more likely undersized — and at risk of overheating and premature failure.
External rotor motors — also called cup motors — are brushless dc motors that differ from traditional motor designs employing permanent magnets. They excel in driving an array of blower and fan applications. Shown here is a disassembled Windjammer PRO with such a design; the monolithic cup rotor and shaft assembly are in the foreground.
Some blowers drive the output impeller by the motor-rotor output using a mechanical coupling or other linkage, and in some cases, even through a speed reducer.
Here, calculations should include equivalency inertia expressions that quantify that seen by the rotor output shaft. In contrast, many AMETEK DFS blowers directly integrate the blower impeller assembly into the rotor output so that their inertia is accurately expressed as a single value. That means design engineers can use expressions for the impeller’s inertia for calculations to determine if its motor will reach application speed sufficiently quickly.
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